The following table summarizes our intangible assets by business segment at the dates indicated:

December 31, 2017

December 31, 2016

Gross

Value

Accumulated

Amortization

Carrying

Value

Gross

Value

Accumulated

Amortization

Carrying

Value

NGL Pipelines & Services:

Customer relationship intangibles

$

447.4

$

(187.5

)

$

259.9

$

447.4

$

(172.7

)

$

274.7

Contract-based intangibles

280.8

(218.4

)

62.4

279.9

(204.4

)

75.5

Segment total

728.2

(405.9

)

322.3

727.3

(377.1

)

350.2

Crude Oil Pipelines & Services:

Customer relationship intangibles

2,203.5

(127.0

)

2,076.5

2,204.4

(84.5

)

2,119.9

Contract-based intangibles

281.0

(171.0

)

110.0

281.0

(121.9

)

159.1

Segment total

2,484.5

(298.0

)

2,186.5

2,485.4

(206.4

)

2,279.0

Natural Gas Pipelines & Services:

Customer relationship intangibles

1,350.3

(417.1

)

933.2

1,350.3

(390.0

)

960.3

Contract-based intangibles

464.7

(379.5

)

85.2

464.7

(370.5

)

94.2

Segment total

1,815.0

(796.6

)

1,018.4

1,815.0

(760.5

)

1,054.5

Petrochemical & Refined Products Services:

Customer relationship intangibles

181.4

(45.9

)

135.5

185.5

(43.9

)

141.6

Contract-based intangibles

46.0

(18.4

)

27.6

54.0

(15.2

)

38.8

Segment total

227.4

(64.3

)

163.1

239.5

(59.1

)

180.4

Total intangible assets

$

5,255.1

$

(1,564.8

)

$

3,690.3

$

5,267.2

$

(1,403.1

)

$

3,864.1

The following table presents the amortization expense of our intangible assets by business segment for the periods indicated:

For the Year Ended December 31,

2017

2016

2015

NGL Pipelines & Services

$

28.9

$

30.6

$

33.6

Crude Oil Pipelines & Services

92.5

98.4

87.1

Natural Gas Pipelines & Services

36.2

33.2

40.0

Petrochemical & Refined Products Services

9.3

9.1

8.9

Offshore Pipelines & Services

--

--

4.5

Total

$

166.9

$

171.3

$

174.1

The following table presents our forecast of amortization expense associated with existing intangible assets for the years indicated:

2018

2019

2020

2021

2022

$

174.5

$

165.8

$

150.7

$

145.6

$

141.6

In general, our intangible assets fall within two categories – customer relationship and contract-based intangible assets. The values assigned to such intangible assets are amortized to earnings using either (i) a straight-line approach or (ii) other methods that closely resemble the pattern in which the economic benefits are estimated to be consumed or otherwise used, as appropriate.